1. Field of the Invention
This invention relates to an ink set, an ink jet recording method, an ink cartridge, a recording unit and an ink jet recording apparatus.
2. Description of the Related Art
In recent years, ink jet recording apparatus have become popular which carry both pigment inks and dye inks and are both achievable of sharply recording images of photographs or the like and sharply recording common documents, web pages, e-mails or the like on recording mediums such as plain paper mainly in monochrome modes. In order to attain a high image quality level when images are recorded on recording mediums such as plain paper by the use of an ink set having a pigment ink and a dye ink, it is demanded to use inks not only affording a high image density but also less causative of blurs. The use of inks less causative of blurs can keep the blurs from being caused at boundaries where images formed using a pigment ink and images formed using a dye ink adjoin one another, i.e., enables formation of images having superior bleeding resistance.
In general, in order to improve image density in images formed using pigment inks, the following are attempted. For example, in order to leave coloring materials on a recording medium in a larger quantity, it is attempted to control the permeability of inks into the recording medium, and, in order to secure a large area where the surface of a recording medium can be covered with coloring materials (the area being what is called an area factor), it is attempted to apply inks in a large quantity.
In such methods, however, it may take a time for the inks to dry, because the permeability of inks has been controlled or the inks are applied in a large quantity. Hence, when images are continuously recorded at a high speed, problems such as set-off may come about during continuous recording or the bleeding resistance may greatly lower. More specifically, during the time that recorded images formed on a second sheet after images have been formed on a first sheet are delivered out of an ink jet recording apparatus, inks of the recorded images on the first sheet may adhere to the back of the second-sheet recording medium because the inks of the images on the first sheet have not well dried (fixed). In addition, where images formed using a pigment ink and images formed using a dye ink form images adjoining one another, a phenomenon may remarkably occur in which the pigment ink blurs to the dye ink, to cause a lowering of the bleeding resistance.
In order to improve the bleeding resistance, it is also attempted to, e.g., improve the permeability of inks into a recording medium to keep the inks from blurring. In this method, however, the coloring materials may less remain on the recording medium to achieve no sufficient image density.
Thus, the improvement in image density and the improvement in bleeding resistance stand a relationship of trade-off each other in many cases. In order to resolve such problems together, studies have hitherto been made on various techniques as stated below. For example, proposals are made on a technique in which the composition of the pigment ink is designed so as to make coloring materials highly associative or aggregative (see Japanese Patent Applications Laid-open No. 2000-198955, No. 2005-206615, No. 2002-167534 and No. 2004-143290). A proposal is also made on a technique in which a pigment ink is reacted with a reaction fluid containing a specific compound, so as to accelerate the aggregation of a pigment (see Japanese Patent Application Laid-open No. 2000-63719). Proposals are still also made on a technique in which the physical properties of a pigment ink are specified so as to control the permeation or blurring of the pigment ink (see Japanese Patent No. 2516218 and Japanese Patent Applications Laid-open No. 2003-238851, No. 2005-200566 and No. 2003-231838). Proposals are still also made on a technique in which the physical properties of a dye ink are specified so as to control the permeation or blurring of the dye ink (see Japanese Patent Applications Laid-open No. S63-213581, No. 2004-83621 and No. H09-296139). A proposal is further made on a technique in which the relationship of physical properties between a plurality of inks is specified so as to control the permeation or blurring of these inks (see Japanese Patent Application Laid-open No. 2006-63322). The background arts enumerated above are further discussed below.
As the technique in which the composition of the pigment ink is designed so as to make coloring materials highly associative or aggregative, a proposal as reported below is made. For example, an ink containing a self-dispersion type carbon black and a specific salt are used so as to achieve improvements in image density and bleeding resistance (see Japanese Patent Application Laid-open No. 2000-198955). In this technique, the pigment present in the ink in a dispersed state is forcedly aggregated on the surface of a recording medium so that the pigment can be kept from permeating into the recording medium, to achieve higher image density and bleeding resistance on images obtained using a conventional pigment ink. In this technique, however, pigment particles are made to aggregate on the surface of the recording medium, and hence the area where the recording medium surface can be covered with coloring materials (what is called “area factor”) may be insufficient as compared with the volume of ink droplets.
To cope with such a problem, a proposal as reported below is made. For example, a proposal is made on that an ink containing a plurality of water-soluble organic solvents and a water-insoluble coloring material is made up as described below, so as to make the coloring material aggregate effectively in the vicinity of the surface of a recording medium even though ink droplets have a small volume, to achieve a sufficient image density (see Japanese Patent Application Laid-open No. 2005-206615). Stated specifically, it is an ink so made up that the plurality of water-soluble organic solvents are a good medium and a poor medium, for a self-dispersion pigment, and a water-soluble organic solvent which shows the maximum Ka value among the respective Ka values of the plurality of water-soluble organic solvents as determined by the Bristow method is the poor medium.
In respect of changes in particle size of a pigment or dispersion stability of the pigment at the time a pigment ink containing water or a water-soluble organic solvent is evaporated, a proposal as reported below is made. For example, a proposal is made on that the increase rate of the average particle size of the pigment is controlled to be 25% or less in a pigment ink 30% by mass of which has been evaporated (see Japanese Patent Application Laid-open No. 2002-167534). A proposal is further made on an ink achievable of dispersion stability even though 50% by mass of a pigment ink has been evaporated (see Japanese Patent Application Laid-open No. 2004-143290).
As the technique in which a pigment ink is allowed to react with a reaction fluid containing a specific compound, so as to accelerate the aggregation of a pigment, a proposal as reported below is made. For example, a proposal is made on that an ink containing a pigment, fine polymer particles, a water-soluble organic compound and water and an aqueous solution containing a polyhydric metal salt are applied to a recording medium and these ink and solution are allowed to react with each other on the recording medium so as to form images with a high quality level (see Japanese Patent Application Laid-open No. 2000-63719).
As the technique in which the physical properties of a pigment ink are specified so as to control the permeation or blurring of the pigment ink, a proposal is made which takes note of dynamic surface tension of ink. For example, a proposal is made on that an ink having been set to be [dynamic surface tension (dyne/cm) at lifetime of 0 millisecond+viscosity (cp)] 42 to 49 can be an ink having superior drying performance (see Japanese Patent No. 2516218). A proposal is also made on an ink in which the maximum value of the rate of change in dynamic surface tension with time is controlled to be from 0.2 mN/m/s or more to 0.4 mN/m/s or less (see Japanese Patent Application Laid-open No. 2003-238851). A proposal is further made on an ink in which the dynamic surface tension (dyne/cm) at a lifetime of 10 milliseconds is from 25 to 50 mN/m and the difference in dynamic surface tension between the maximum value and the minimum value is 5 mN/m or less (see Japanese Patent Application Laid-open No. 2005-200566). A proposal is still further made on an ink in which the dry viscosity is 100 mPa·s or less, the dynamic surface tension (dyne/cm) at a lifetime of 10 milliseconds is 45 mN/m or more and the dynamic surface tension (dyne/cm) at a lifetime of 1,000 milliseconds is 35 mN/m or less (see, e.g., Japanese Patent Application Laid-open No. 2003-231838).
As the technique in which the physical properties of a dye ink are specified so as to control the permeation or blurring of the dye ink, a proposal is made which takes note of static surface tension (what is called “surface tension”) of the ink and dynamic surface tension of the ink. As to the former, a proposal is made on that the surface tension of ink at 20° C. is controlled to be 45 mN/m or more so as to make small the area per one dot the ink forms, to keep the ink from blurring on a recording medium (see Japanese Patent Application Laid-open No. S63-213581). A proposal is also made on that the surface tension of ink is controlled to be 40 mN/m or more so as to keep the ink from blurring (see Japanese Patent Application Laid-open No. 2004-83621). As to the latter, a proposal is made on that the surface tension of ink at an air-bubble period T (second/air bubble) of ≦0.2 is controlled to be 40 mN/m or more so as to keep the ink from blurring, and the surface tension at T>1 is controlled to be less than 50 mN/m so as to improve the ink in its ejection reliability (see Japanese Patent Application Laid-open No. H09-296139). In the above Japanese Patent Application Laid-open No. 2003-231838, it is also disclosed that the dynamic surface tension at a lifetime of 10 milliseconds is controlled to be 45 mN/m or more so as to keep the ink from blurring and the dynamic surface tension at a lifetime of 1,000 milliseconds is controlled to be 35 mN/m or less so as to improve the ink in its quick-drying performance.
As the technique in which the relationship of physical properties between a plurality of inks are specified so as to control the permeation or blurring of these inks, a proposal as reported below is made. For example, a proposal is made which takes note of the relationship of dynamic surface tension in a plurality of inks making up an ink set (see Japanese Patent Application Laid-open No. 2006-63322). More specifically, in an ink set consisting of a black ink, a yellow ink, a magenta ink and a cyan ink, the dynamic surface tension of each ink in the same lifetime within the range of lifetimes of from 30 milliseconds to 1,000 milliseconds at a temperature of 25° C. as measured by a maximum bubble pressure method is specified in the following way: (1) The yellow ink has a higher dynamic surface tension than the black ink. (2) The difference in dynamic surface tension between the yellow ink and the black ink is 5 mN/m or less. (3) The difference in dynamic surface tension between the yellow ink and the magenta ink and the difference in dynamic surface tension between the yellow ink and the cyan ink are both 3 mN/m or more. (4) The magenta ink and cyan ink each have a lower dynamic surface tension than the black ink. It is disclosed that specifying the dynamic surface tension in these ways brings an improvement in bleeding resistance.
The present inventors have investigated advantages and problems any conventional inks have had, and have analyzed characteristic features of images obtained using such inks. As the result, they have found that the rate at which the ink permeates into a recording medium after an ink having been ejected out of a recording head has adhered to the recording medium and the manner of a change in condition (aggregation) of the ink differs depending on the type of the recording medium. Here, the rate at which the ink permeates into the recording medium refers not to the rate at which it diffuses through the interior of the recording medium, but to the rate at which the ink comes not to exist on the surface of the recording medium after the ink has been applied to the recording medium.
That is, the background arts for achieving a high image density have mainly aimed to control the state of a coloring material. Accordingly, where an ink the coloring material of which aggregates concurrently as the ink permeates into the recording medium is applied to a recording medium having a high ink permeation rate, the coloring material may insufficiently come to aggregate during the time the ink permeates into the recording medium, and hence no sufficient image density has been achievable in some cases. Besides, where images are formed in which regions of images formed using a pigment ink and regions of images formed using a dye ink adjoin one another, the pigment may insufficiently come to aggregate during the time the pigment ink permeates into the recording medium, to tend to come into contact with the dye ink, and hence bleeding has occurred in some cases.
To cope with such a problem, it is attempted as in the above Japanese Patent Application Laid-open No. 2005-206615 to bring a water-insoluble coloring material and a plurality of water-soluble organic solvents into a specific relationship, stated specifically, to use as a poor medium for the water-insoluble coloring material a water-soluble organic solvent having the maximum Ka value determined by the Bristow method. This makes the coloring material diffuse and aggregate effectively in the vicinity of the surface of a recording medium even though ink droplets have a small volume, to achieve sufficient image density and bleeding resistance, as so reported. In this technique, however, after the ink has been applied to the recording medium, the coloring material is made to aggregate while it is made to diffuse in the vicinity of the surface of the recording medium. Hence, any coloring material that has not come to aggregate at the time the coloring material begins to diffuse permeates into the recording medium in its thickness direction. As the result, in a recording medium having a high ink permeation rate, it has sometimes come about that any high image density is not obtainable and the bleeding can not be kept from occurring. This means that, where in a recording medium it differs in surface state or in wettability to inks, the ink differs greatly in its permeation rate and diffusion rate to the recording medium. Then, when images are formed on such a recording medium having a high ink permeation rate, the aggregation of the coloring material comes not to catch up the permeation and diffusion of the ink, so that the coloring material in the ink permeates into the recording medium in its thickness direction. As the result, depending on the type of the recording medium, the problems may come about such that no sufficient image density and bleeding resistance are achievable.
In both the above Japanese Patent Applications Laid-open No. 2002-167534 and No. 2004-143290, what is taken into account is limited only to how the pigment in the pigment ink behaves after its dispersion has come to stable regions or dispersion has become unstable until it begins to aggregate, and is insufficient for well improving the image density and the bleeding resistance. In the above Japanese Patent Application Laid-open No. 2000-63719, a pigment ink and an aqueous solution containing a polyhydric metal salt are allowed to react with each other on the recording medium. However, components making up an ink and recording systems are demanded to be more simplified. That is, it is demanded that inks are made less dependent on external factors so as to keep the inks from the difficulty of a lowering of reliability or the like that is caused by complex action, to provide inks in which coloring materials, water-soluble organic compounds and so forth have appropriately be designed.
As in the above Japanese Patent No. 2516218 and Japanese Patent Applications Laid-open No. 2003-238851, No. 2005-200566 and No. 2003-231838, some proposals are made on inks taking not of dynamic surface tension. All of these techniques, however, do not at all take account of what changes in condition the coloring material has caused in the course that the ink comes to permeate after it has been applied to the recording medium. That is, in the techniques disclosed in Japanese Patent No. 2516218 and Japanese Patent Applications Laid-open No. 2003-238851, No. 2005-200566 and No. 2003-231838, the phenomenon being taken place in the recording medium is not taken into account, and hence any high image density and bleeding resistance the present inventors seek are not achievable by merely specifying ink physical properties as in these publications. For example, in the ink disclosed in Japanese Patent No. 2516218, the value is specified that is found by adding the values of dynamic surface tension at a lifetime of 0 millisecond and viscosity. In the inks disclosed in Japanese Patent Applications Laid-open No. 2003-238851, No. 2005-200566 and No. 2003-231838, the dynamic surface tension at a lifetime of 10 millisecond and the value at which the dynamic surface tension changes are specified. Accordingly, the present inventors have prepared an ink which satisfies the conditions disclosed in Japanese Patent No. 2516218 and Japanese Patent Applications Laid-open No. 2003-238851, No. 2005-200566 and No. 2003-231838, and have studied image density and bleeding resistance brought by such an ink. However, it has been found unable to achieve any image density and bleeding resistance at the level the present inventors have sought. This is because the ink does little come to evaporate at the lifetime of 0 millisecond or 10 milliseconds, and the ink has not come to change in condition to such an extent as to enhance image density and bleeding resistance on the recording medium. That is, the techniques disclosed in Japanese Patent No. 2516218 and Japanese Patent Applications Laid-open No. 2003-238851, No. 2005-200566 and No. 2003-231838 do not at all take account of the changes in condition of ink on the recording medium, and hence the dynamic surface tension at the lifetime of 0 millisecond or 10 milliseconds can be said to be a value having no significance at all in regard to how the image density and bleeding resistance be improved.
In addition, the value [dynamic surface tension (dyne/cm) at lifetime of 0 millisecond+viscosity (cp)]=42 to 49 in the above Japanese Patent No. 2516218 is too low to keep the ink from permeating into, in particular, a recording medium with coarse fiber and many voids. Similarly, the value that the dynamic surface tension at a lifetime of 10 milliseconds is 45 mN/m as in the above Japanese Patent Application Laid-open No. 2003-231838 is also too low to keep the ink from permeating into, in particular, such a recording medium with coarse fiber and many voids. From these points as well, the techniques disclosed in Japanese Patent No. 2516218 and Japanese Patent Application Laid-open No. 2003-231838 can be said to be unable to improve the image density and the bleeding resistance.
In the case when the images are formed in which images formed using a pigment ink and images formed using a dye ink adjoin one another, it is considered that the dye ink blurs so greatly as to tend to come into contact with the pigment ink, to cause bleeding. Accordingly, in respect of the bleeding resistance, it is important not only to control aggregative properties, blurring and permeation of the pigment ink, but also to take an approach from the dye ink.
However, in the dye ink as well, like the technique disclosed in Japanese Patent Application Laid-open No. S63-213581 or No. 2004-83621, it may be unable to control the blurring by merely making the ink have a high surface tension. This is because, if the ink continues to have a high surface tension for a long time, the permeation of ink into the recording medium does not proceed, so that the ink may spread on the surface of the recording medium. In addition, if the dye ink is made to have a high surface tension, it can not wet the surface of the recording medium uniformly, and hence the uniformity of colors in images may lower. Besides, if the ink is made to have a high static surface tension, the ink comes less permeable into the recording medium, and hence it may take a long time until the ink permeates completely into the recording medium, so that ink set-off or the like may occur.
In the techniques disclosed in Japanese Patent Applications Laid-open No. 2003-231838 and No. H09-296139, it is attempted to control blurring and permeation, taking note of the dynamic surface tension at a lifetime of 10 milliseconds and changes in the dynamic surface tension. However, studies made by the present inventors have revealed that any bleeding resistance at the level the present inventors seek is not achievable at least by specifying the dynamic surface tension as disclosed in the above Japanese Patent Applications Laid-open No. 2003-231838 and No. H09-296139. This is presumed due to the fact that the ink has little come to evaporate, or permeate into the recording medium at its surface regions, at least at the lifetime of 10 milliseconds and the dot area has not been made controllable even by specifying the value of dynamic surface tension at the lifetime of 10 milliseconds. That is, in the techniques disclosed in Japanese Patent Applications Laid-open No. 2003-231838 and No. H09-296139, the changes in condition in ink on the recording medium is not taken into account, thus the dynamic surface tension at a lifetime of 10 milliseconds can be said to be a value having no significance at all in regard to how the blurring be controlled and how the bleeding resistance be improved.
Further, in the technique disclosed in Japanese Patent Application Laid-open No. 2006-63322, the relationship of dynamic surface tension within the range of lifetimes of from 30 milliseconds to 1,000 milliseconds between a black ink and color inks is specified. Stated specifically, the dynamic surface tension of a yellow ink and a black ink each in the same lifetime within the range of lifetimes of from 30 milliseconds to 1,000 milliseconds is so specified that the yellow ink has a higher dynamic surface tension than the black ink. In virtue of this, when these inks come into contact with each other on the recording medium, the permeation of the yellow ink is pulled by the permeation of the black ink, where images formed using the black ink are color-mixed with the yellow ink, greatly different in color value, and this can keep the bleeding from occurring, as so reported. Accordingly, the present inventors have prepared an ink which satisfies the requirements disclosed in Japanese Patent Application Laid-open No. 2006-63322, and have studied image density and bleeding resistance in images formed by applying to a recording medium the ink obtained. As the result, it has been found that any image density and bleeding resistance at the level the present inventors seek is not achievable. As to the reason therefor, the present inventors consider it as stated below. Relatively large voids are present in a large number in plain paper, and hence, as the inks are applied to make the surface of plain paper wet, the inks are taken into the voids, where the movement of the inks is restricted to a certain extent. Hence, the influence the relationship of physical properties in a plurality of inks has on the bleeding resistance is limitative. In an ink set made up of a plurality of inks, in order to control the bleeding, it is a very important factor to appropriately determine the wettability of each ink to plain paper and the extent of blurring in the above each ink. That is, the bleeding can not be controlled by merely specifying the relationship of dynamic surface tensions in a plurality of inks making up the ink set as in the case of Japanese Patent Application Laid-open No. 2006-63322.